The cornea is the transparent dome-shaped front part of our eyes and their most important focusing lens. Like the lens of a camera, its surface must be perfectly smooth in order to provide clear vision. When disease or injury causes the corneal surface to become irregular, the eye can no longer focus clearly, even with the strongest glasses. Hard contact lenses have the unique ability to improve the vision of these eyes by creating a smooth layer of tears that optically masks the irregular surface of the cornea. However, there are many eyes with damaged corneas that cannot be fitted with a hard contact lens that rests on their corneas. Moreover, the corneas of patients who suffer from severe ocular surface disease become so exquisitely fragile that they are often unable to withstand the pressure of blinking or the briefest exposure to air and even less so the friction of a hard contact lens.
A scleral lens, such as the Boston Scleral Lens offered by the Boston Foundation for Sight, provides a device that addresses this condition. As shown in FIGS. 1-3 and 3A, a scleral lens 10, which is about the size of a quarter, rests on the relatively insensitive white sclera 12 of the eye 14 and creates a space 16 over the cornea 18 that is filled with artificial tears. The eye contact location for the scleral lens is different than conventional contact lenses which rest on the cornea. As illustrated by the contact length L in FIG. 2, contact with the eye is limited to the sclera and the lens does not contact the cornea 18. By creating a fluid-filled space over the irregular surface of the damaged cornea, this lens device can be helpful in improving vision in eyes with extremely distorted corneas. Moreover, this fluid compartment becomes a liquid bandage that protects the raw and sensitive cornea from exposure to air and the rubbing effects of blinking. The therapeutic environment created between the lens and the cornea nurtures healing and can virtually eliminate pain and photosensitivity. It is this unique “corneal liquid bandage” that is responsible for the extraordinary healing experienced by patients who have used the scleral lens. However, by sealing a pool of liquid over the cornea, the scleral lens simulates a fluid pump by which micro quantities of fluid are squeezed out of the fluid compartment during lens compression as occurs during blinking. When the lens is decompressed after each blink, tears outside the lens are drawn into the fluid compartment. If the aspiration of tears is blocked during lens decompression, negative pressure develops in the fluid compartment that increases over time resulting in progressive lens suction that can be severely damaging to the eye. The traditional solution to scleral lens suction is to drill a hole in the lens through which air is aspirated (air ventilated). However, the presence of air bubbles in the fluid compartment causes desiccation of the corneal surface that is especially dangerous for diseased corneas. The purpose of this invention is to create a mechanism for preventing lens suction by facilitating the aspiration of outside tears while excluding the transit of air bubbles.
A groove 22 is provided in that part of the posterior contact lens surface that rests on the scleral surface of the eye (haptic). More than one groove is typically used. The groove(s) allows tears outside the lens to be aspirated into the central fluid compartment to prevent the development of suction.
One of the reasons for the effectiveness of the Boston Scleral Lens is its highly oxygen-porous plastic, developed under the leadership of Boston Foundation for Sight founder Dr. Perry Rosenthal, that allows the cornea to breathe through the lens. The cornea, unlike any other surface tissue of the human body breathes by extracting oxygen directly from the surrounding air rather than from the blood circulation and the oxygen porous plastic allows oxygen to reach the surface of the eye.
A scleral lens can be beneficial to treat a number of conditions. Among the most common conditions treated is severe dry eye. Some causes of dry eyes are an underlying medical condition, such as Sjogren's syndrome, graft versus host disease (following bone marrow transplants), radiation treatment in the eye area, Stevens-Johnson syndrome, and autoimmune disorders. In some cases the cause is unknown (idiopathic). The condition can be due to a diminished supply of tears, excessively rapid evaporation or both. Dry eyes is one of the most common ocular complaints. Symptoms vary from mildly annoying, intermittent dryness and burning and paradoxical tearing (worse in dry environments) to constant disabling pain, increased light sensitivity and blurred vision. Mild to moderate cases of dry eyes can usually be controlled by the frequent use of artificial tears, punctal plugs that slow the drainage of tears from the eyes and a new, prescription eye drop (Restasis) that may reduce the inflammation associated with dry eyes. This group constitutes the vast majority of dry eyes. However, it is the patient with severe, disabling dry eyes unresponsive to the above treatments who can benefit the most from the Boston Scleral Lens device (BSL) and for whom scleral lens is being prescribed.
It is important to treat severe dry eyes, which is one of the conditions known as severe ocular surface disease. Such patients suffer constant disabling eye pain and photosensitivity and may develop erosions on the surface of their corneas that impair vision and can lead to scarring and permanent impairment of vision. In the most severe cases, corneal ulcers develop that are slow to heal (if they heal at all) and can result in perforation. Rarely, it may be necessary to suture the lids together in order to save the eye (tarsorrhaphy). These severely dry eyes are more prone to infection and heal so poorly that surgery, including corneal transplant, is rarely successful and even dangerous. Conventional treatments described above are of little value. The so-called soft bandage lens is often ineffective because it requires an adequate supply of tears to maintain its hydration.
Keratoconus is another eye condition can be addressed by the use of a scleral lens. Keratoconus is characterized by a progressive thinning and steepening of the central cornea. As the cornea steepens and thins, a patient experiences a decrease in vision which can be mild or severe depending on the severity of the disease. Keratoconus has no known cure.
Onset of keratoconus occurs during the teenage years—mean age of onset is age 16 years—but onset has been reported to occur at ages as young as 6 years. Keratoconus shows no gender predilection and is bilateral in over 90% of cases. In general, the disease develops asymmetrically: diagnosis of the disease in the second eye lags about five years after diagnosis in the first. The disease process is active for about five to 10 years, then it may be stable for many years. During the active stage, change may be rapid.
Typically, early vision loss can be corrected by spectacles; later, irregular astigmatism requires optical correction with rigid contact lenses. Hard contact lenses provide a uniform refracting surface and therefore improve vision. However, traditional rigid contact lenses rest on the surface of the cornea and slide over this surface with each blink. As a result, patients with diseased or damaged corneas who are most likely to benefit from their unique vision-restoring properties are often unable to tolerate them and would face the potential serious complications and uncertain visual outcome of corneal transplant surgery.
An estimated hundreds of thousands of patients suffering from corneal disorders in the U.S. alone can benefit from the Foundation's lenses. The vast majority of patients are young-some are children. The Boston Scleral Lens is their only hope for regaining function vision. For the remaining patients, our lenses replace the need for corneal transplant surgery. The cornerstone of the Boston Scleral Lens is the liquid bandage it sustains over the corneal surface. This device is the first fluid-ventilated gas-permeable scleral lens designed to maintain an oxygenated aqueous corneal environment free of air bubbles. Lens suction is avoided by incorporating a series of radial channels in the posterior haptic surface that facilitates the aspiration of tears (while excluding air) as the means of aborting the development of negative hydrostatic pressure.
In order to avoid obstructing the channels (a prerequisite to the physiological tolerance of the devices), the shape of each haptic bearing surface is adapted to the contour of the underlying sclera through the design/fitting process. This flexibility and precision has been made possible by a process described in U.S. Pat. No. 5,452,031, the entire contents of which is incorporated by reference. Based on advanced mathematical functions known as “splines”, the design program has been integrated with the control system of the state-of-the-art contact lens lathe. This on-site technology has been an essential resource for advancing the development of the Foundation's lenses and enabling us to reach a success rate exceeding 90%. The oxygen tension of the corneal liquid bandage is maintained by a highly oxygen permeable polymer from which the devices are fabricated. Specially manufactured discs of this material are made for the Boston Scleral Lenses by Bausch & Lomb.
The process of making and adapting the shape of scleral lenses to that of the individual eye is exacting and time consuming. Additionally, various channels can be used to enable tears outside the lens to be aspirated into the liquid reservoir between the lens and cornea to abort the development of lens suctions, a highly dangerous situation. At the same time, it is necessary to design the channels to avoid the aspiration of air into the fluid reservoir that would adversely affect the health of the cornea. In order to maintain the patency of the channels, it is necessary to match the shape of the bearing surface of the scleral lens (haptic) with that of the underlying eye surface (sclera) in order to avoid excessive eye compression that would obstruct the channels and render them ineffective. It would be beneficial to enhance the performance of the scleral lens by configuring the groves in manner that makes them less vulnerable to compression or enabling the transit of air bubbles and thereby reduce the precision now required in customizing the shape of their haptic bearing surface.